Transformers and balanced-to-unbalanced (BALUN) devices are commonly used in wireless communications. For example, transformers and BALUNS are frequently used in transceivers in wireless communication devices as illustrated in FIGS. 11 and 12. As shown in FIG. 12, the modulator includes a conventional BALUN K0 having a center tap CTP connected to Vdd1. Conventional coplanar interleaved transformers used in such applications have the primary and secondary windings interleaved on the same integrated circuit layer. The primary and secondary windings are constructed of planar metal traces.
FIG. 1A illustrates a conventional coplanar symmetric transformer 100 having a 2:2 turn ratio. As shown in FIG. 1A, the transformer 100 includes a primary winding 102 and a secondary winding 106, which are both located on the same metal layer. The segments 102a, 102b, 102c, and 102d that comprise primary winding 102 are connected by metal bridges 104a, 104b, and 104c formed on a separate metal layer and connecting vias (not shown). Similarly, the secondary winding 106 is comprised of segments 106a, 106b, 106c, and 106d that are formed on the same metal layer as the segments comprising the primary winding 106. Additionally, metal bridges 108a, 108b, and 108c that connect the segments 106a, 106b, 106c, and 106 that form secondary winding 106 are formed on the same metal layer on which metal bridges 104a, 104b, and 104c are formed. While conventional coplanar interleaved transformers reduce the size and resistance, they suffer from low quality (Q) factors and small coupling coefficients.
FIG. 1B illustrates a conventional coplanar BALUN device 150 having a 2:2 turn ratio. BALUN device 150 includes a primary winding 152 and a secondary winding 156. Primary winding 152 includes winding segments 152a, 152b, 152c, 152d, and 152e, which are formed on a first metal layer and are connected by metal bridges 104a, 104b, and 104c formed on a second metal layer. Center tap 160 is formed on a third metal layer and connects primary winding segments 152b and 152c. 
Secondary winding 156 is also formed from a plurality of winding segments 156a, 156b, 156c, 156d, and 156e formed on the same metal layer as primary winding 152. Secondary winding segments 156a-e are connected by metal bridges 158a, 158b, and 158c located on the same metal layer as metal bridges 154a-c. Center tap 162 is formed on the same metal layer as center tap 160 and connects winding segments 156b and 156c. 
As shown in FIG. 1B, the center taps 160 and 162 are connected at the inner-most portion of the BALUN 150 and extend to the outer-most portion. This center tap location is an undesirable location as it may result in high current density on the center taps 160, 162, which may have a detrimental affect on the reliability of the circuit due to electromigration. For example, the reliability of a transceiver circuit as illustrated in FIG. 12 would be detrimentally affected if a BALUN 150, as illustrated in FIG. 1B, were implemented as the BALUN in FIG. 12 as the location of the center tap 160, e.g., CTP in FIG. 12, would have a high current density. Additionally, if the BALUN illustrated in FIG. 12 included a center tap on the secondary winding (CTS) as shown in FIG. 1B, e.g., center tap 162 in FIG. 1B, the reliability of the transceiver circuit would be reduced.
Accordingly, improved transformers and BALUNs are desired.